Known injection devices are shown in WO 95/35126 and EP-A-0 516 473 and tend to employ a drive spring and some form of release mechanism that releases the syringe from the influence of the drive spring once its contents are supposed to have been discharged, to allow it to be retracted by a return spring.
Often, such injection devices are required to work with sealed hypodermic syringes which typically have a hermetically sealed cover or “boot” that covers the hypodermic needle and maintains the sterility of the syringe contents. Naturally, it is necessary to maintain the sterility of the syringe contents up to the point of administration, which means that for devices that are designed to be disposable, the boot must be removed with the syringe inside the injection device.
Generally, the action required to remove the boot from the syringe entails either pulling the boot away from the syringe or twisting the boot and, either at the same time or subsequently, pulling the boot away from the syringe.
Co-pending United Kingdom patent application no. 0412051.5, the contents of which is incorporated herein by reference, describes a cap for an injection device that connects to the boot of the syringe so that removal of the housing closure member from the housing causes removal of the boot from the syringe.
In certain types of syringe, for example a Bünder ReadyJect™ type syringe, the boot must be twisted to break a frangible joint before it can be pulled off. Since the syringe is contained in the housing and not held directly by a user, it must be supported rigidly in the housing so that rotation of the boot does not cause rotation of the syringe which would result in the boot not being removed. This is particularly the case when significant rotational force is required to remove the boot and/or break the frangible joint.
The cylindrical shape of a syringe does not lend itself to being supported against rotation, especially since the syringes which are used with injection devices of the type described herein are often made of glass which can fracture.